(Refactor) Move the Render trait into mod.rs instead of submodule

src/renderer/mod.rs

@@ -1,5 +1,9 @@
-pub use self::renderer::Renderer;
 pub use self::html_handlebars::HtmlHandlebars;
 
-pub mod renderer;
 mod html_handlebars;
+
+use std::error::Error;
+
+pub trait Renderer {
+    fn render(&self, book: &::book::MDBook) -> Result<(), Box<Error>>;
+}

src/renderer/renderer.rs

@@ -1,5 +0,0 @@
-use std::error::Error;
-
-pub trait Renderer {
-    fn render(&self, book: &::book::MDBook) -> Result<(), Box<Error>>;
-}